1. Field of the Invention
The present invention relates to General Packet Radio Service Networks; and more particularly, link adaptation in such networks.
2. Description of Related Art
To exploit the wide range of carrier-to-interference (C/I) ratios experienced over the radio interface, General Packet Radio Service (GPRS) networks employ four different airlink coding schemes. An airlink coding scheme is an error correction code that generates a total of n encoded bits for each k information bits, and therefore, has a coding rate
      k    n    .CS-1 is GPRS's lowest rate code. CS-4 is GPRS's highest rate code. In the set containing codes {CS-1, CS-2, CS-3}, CS-3 is the highest rate code, CS-1 is the lowest rate code.
One coding scheme is stronger than another if it is capable of correcting more channel errors per block. CS-1 is stronger than CS-2, CS-3 and CS-4. Similarly, CS-2 is weaker than CS-1, since CS-2 is capable of correcting fewer channel errors than CS-1.
GPRS's lowest rate code, CS-1, employs a relatively high number of redundancy bits and offers a maximum logical link control (LLC)-layer throughput of 8 kbps/timeslot. The high level of redundancy present in blocks encoded using CS-1 ensures that mobile stations at the fringes of a cell, where C/I levels are typically lowest, are able to send and receive data. In contrast, the highest rate code, CS-4, offers maximum LLC-layer throughputs of 20 kbps/timeslot. Because of the small number of redundancy bits added to each block encoded with CS-4, however, airlink errors can be detected, but not corrected. As a result, CS-4 offers the best airlink performance at relatively high C/I ratios. The remaining two GPRS coding schemes offer maximum LLC-layer throughputs of 12 kbps/timeslot (CS-2) and 14.4 kbps/timeslot (CS-3).
By monitoring the quality of the airlink, the GPRS network can select the coding scheme that offers the best performance. The process of dynamically selecting the coding scheme based on airlink quality is called link adaptation.
A link adaptation algorithm, for example, may wish to select the coding scheme that maximizes link layer throughput, subject to block error rates falling below a desired maximum target (˜20%). Such a performance measure strikes a balance between the goal of airlink efficiency (high throughputs) with the goal of keeping delay variance tolerable (low block error rates).
However, link adaptation decisions based on maximizing throughput alone neglects the debilitating effects high block error rates have on higher layer protocols. High block error rates can cause a block to be retransmitted several times before it is correctly received. Highly variable delay in the transmission of radio link control (RLC) blocks can cause RLC flow control windows to stall, throttling the rate at which data can be delivered between the GPRS network and a mobile. Variable delays also wreak havoc on connection-oriented protocols such as TCP, triggering retransmissions which waste airlink resources. High delay variance is also annoying to end users.